Electromagnetic wave radiators



April 19, 1960 K. FOSTER ET AL ELECTROMAGNETIC WAVE RADIATORS Filed.NOV. 28, 1955 jNl/E'NTOES new W 6. 7M

A TTORNE Y United States Patent ELECTROMAGNETIC WAVE RADIATORS KennethFoster, Cockfosters, and Alan Philip Craven Thiele, London, England,assignors to A. C. Cossor Limited, London, England Application November28, 1955, Serial No. 549,486

Claims priority, application Great Britain December 8, 1954 7 Claims.(Cl. 343-756) The present invention relates to electromagnetic waveradiators for use particularly but not exclusively in radar systems.

Radar systems operating at very short wavelengths, for example 3 cms.,suffer from a disadvantage that target indications are sometimesobscured by unwanted indications caused by reflections from rain.

To overcome this disadvantage it has been proposed to provide a radarsystem in which the radiated waves are approximately circularlypolarised. The polarisation of such waves reflected from rain issubstantially unaltered whereas the polarisation of such waves reflectedfrom targets such as aircraft is markedly elliptical. At a receiver inthe system means are provided for discriminating against the circularlypolarised reflected waves.

One object of the present invention is to provide an improvedelectromagnetic wave radiator whereby approximately circularly polarisedwaves can be radiated.

According to the present invention an electromagnetic wave radiator forradiating approximately circularly polarised waves, comprises a horn ofelectrically conducting material and of rectangular cross section, andmeans whereby there can be fed into the throat of the horn in effect twoorthogonal plane polarised waves of like wavelength A, the waves beingpolarised in directions substantially parallel and perpendicularrespectively to one edge of the throat of the horn, the throat and mouthof the horn being of different rectangular shapes and the dimensions ofthe horn being related to A in such a manner that, in operation, one ofthe waves in travelling from the throat to the mouth of the horn isdelayed by approximately nx/4 relatively to the other wave where n is anodd integer. Thus at the mouth of the horn the waves are approximatelyin phase quadrature with one another and hence an approximatelycircularly polarised wave is produced.

In a preferred form of the invention the throat of the horn is of squareshape, and the smaller dimension of the mouth of the horn is equal tothe length of one edge of the throat.

The invention will now be described, by way of example, with referenceto the accompanying drawings, in which Fig. '1 shows a horn radiatorwhereby approximately circularly polarised electromagnetic waves can beradiated,

Fig. 2 shows an assembly of a horn radiator as shown in Fig. 1 togetherwith means for feeding electromagnetic waves into the horn, and

Figs. 3a, 3b, 4, 5 and 6 show alternative arrangements respectively of apart shown in Fig. 2.

Referring to Fig. 1, this shows a horn radiator 10 of rectangularcross-section. The horn may conveniently be of copper. The throat of thehorn is of square crosssection each edge of the throat having the lengtha, and

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the mouth of the horn is of rectangular cross-section one dimensionbeing a and the other s which is greater than a. Thus the horn is flaredin only one dimension. Any suitable means may be provided for feedinginto the throat of the horn two orthogonal, plane polarised waves oflike wavelength A, one of the waves being polarised in a directionparallel to the upper and lower edges (in the drawing) of the throat asshown by the vector E and the other wave being polarised in a directionparallel to the vertical edges of the throat as shown by the vector EThe half-angle of the flare is 00.

It can be shown that if the radiation from the horn along the line ofmaximum gain is substantially circularly polarised.

In Equation i In one example for use with waves of a wavelength of 3.2cms. a=l inch, s=6 inches and the length of the horn is 11.3 inches. Ahorn of these dimensions when used at a wavelength of 3.2 cms. producesa relative delay between the two waves of 7M4.

For the purpose of feeding into the throat of the horn two orthogonalplane polarised waves polarised as shown by the vectors E and E a numberof alternative arrangements have been devised, each having the generalform shown in Fig. 2.

In Fig. 2 the horn 10 is connected to a waveguide 11 of rectangularcross-section through three sections of waveguide 12, 13 and 14respectively. The centre section 13 is of circular cross-section and oneend of the section 14 square and is fitted to the throat of the horn andthe other end is circular and is fitted to one end of the circularsection 13. One end of the section 12 is circular and is fitted to theother end of the section 13. The other end of the section 12 isrectangular and is fitted to the waveguide 11.

In one arrangement having the general form shown in Fig. 2 the section13 of Waveguide of circular crosssection is fed from the waveguide 11with a plane polarised wave in the H mode. The angular position of thewaveguide 11 about its longitudinal axis is made such that the waveemerging from the section 13 and passing through the section 14 into thethroat of the horn 10 is polarised with its E vector at 45 to the edgesof the throat of the horn. This wave is resolved at the throat of thehorn into the two waves required.

In another arrangement the section 13 of waveguide contains a phaseshifter by means of which two H waves polarised at right angles to oneanother are produced. The two waves combine at the output end of thephase shifter to provide a wave polarised with its E vector at 45 to theedges of the throat of the horn. The wave is resolved at the throat ofthe horn into the two waves required.

The phase shifter can take various forms of which examples are shown inFigs. 3, 4 and 5 respectively.

In Fig. 3(a) the section 13 of waveguide contains a strip 15 ofdielectric material. The two ends of the strip are tapered as shown forimpedance matching. Referring to Fig. 3(b) this shows the orientation ofthe plane of the strip 15 relatively to the rectangular waveguide 11 andthe throat of the horn. The angle between the plane of the strip 15 andthe shorter sides of the waveguide 11 is made 22 /2 and the wave in therectangular section is arranged to be in the H mode. On entering thecircular section 13 the wave changes to the H mode and is split into twoH waves Whose E vectors are respectively parallel and perpendicular tothe plane of the strip 15. The length of the strip 15 is chosen to besuch that the wave whose E vector is parallel to the plane of the stripis delayed M2 relatively to the other wave. The two waves combine at thehorn end of the section 13 and produce the plane polarised wave rotatedthrough 45.

In Fig. 4 there is shown an alternative to the arrangement of Fig. 3. InFig. 4 two metal fins 16 and 17 are used, the cut-away portions at theends of the fins providing M4 transformers for impedance matchingpurposes. The orientation of the plane of the fins is made the same asthe dielectric strip 15 shown in Fig. 3(b).

Yet another arrangement is shown in Fig. 5 in which the dielectric stripof Fig. 3 is replaced by a metal plug 18.

The use of the dielectric strip 15, the fins 16, 17 and the plug 18 canbe avoided if the central region of the section 13 is made of ellipticalcross-section. This can be achieved by means of a section of waveguideof circular cross-section provided with a clamp whereby the centralregion of the section can be squeezed into approximately ellipticalshape. Referring to Fig. 6, this shows the orientation of the ellipticalregion relatively to the rectangular waveguide and the throat of thehorn. The major axis of the ellipse is arranged to be at 22 /z to theshorter sides of the waveguide 11. In practice the clamp is adjusted foroptimum conditions.

In any arrangement according to the invention circularly polarised wavescan be generated only along the line of maximum gain of the horn. Toobtain circular polarisation along the line of maximum gain theamplitudes of the two waves fed into the throat of the horn must beexactly equal and one must be delayed relatively to the other by exactlyrun/4 where n is an odd integer. If the amplitudes are unequal or if thedelay is not precisely nA/4 the radiated wave is elliptically polarised.It has been found, however, that a slightly elliptically polarised waveis more suitable for discriminating against rain than a truly circularlypolarised wave. In practice an operator views an indicator and adjuststhe ellipticity for maximum discrimination against rain.

In operation the use of approximately circularly polarised waves fordiscriminating against rain leads to losses and a substantial weakeningof the wanted indications. It is desirable therefore that the systemshould be readily adjustable to enable either approximately circularlypolarised waves or plane polarised waves to be used. The circularlypolarised waves need then be used only when rain is present and at allother times plane polarised waves may be used.

Any of the foregoing arrangements may readily be adapted for thispurpose. In the first described arrangement the waveguide 11 may be maderotatable about its longitudinal axis between a first position in whichplane polarised waves are radiated and a second position in whichapproximately circularly polarised waves are radiated. When in the firstposition the broader walls of the waveguide 11 are parallel to thebroader walls of the horn.

In the arrangements described with reference to Figs. 3(a) and 3(b) andFig. 4, the section 13 of waveguide is arranged to be rotatable from theposition described to a second position in which the strip 15 or thefins 16 and 17 as the case may be are parallel or perpendicular to thenarrower walls of the waveguide 11.

Likewise in the arrangements described with reference to Figs. 5 and 6the section 13 of waveguide is made rotatable between two appropriatepositions,

Rotation of the waveguide 11 or the section 13 can be effected by remotecontrol in any suitable manner. For example the section 13 may bemounted in ball bearings and a spring and stop member may be providedwhich normally position the section 13 to an angular setting in whichplane polarised waves are radiated. A lever may have one end attached tothe section 13 and the other to the armature of a solenoid. When thesolenoid is energised it can be arranged that movement of the armatureof the solenoid and hence the lever causes the section 13 to be rotatedto a second angular setting determined by a second stop member. When inthe second angular setting approximately circularly polarised waves aretransmitted.

Although embodiments of the invention have been described in which thehorn has a throat of square crosssection it will be understood that thethroat may have other rectangular shapes. The criteria determining thedimensions of the horn are that the horn must permit the transmissiontherethrough of both waves fed into the throat of the horn and that oneof the waves must be delayed by approximately n)-/4 relatively to theother wave.

We claim: I t

1. An electromagnetic wave radiator for radiating approximatelycircularly polarised waves, comprising a horn of electrically conductingmaterial and of rectangular cross section, and means for feeding intothe throat of the horn in effect two orthogonal plane polarised waves oflike wavelength A, the waves being polarised in directions substantiallyparallel and perpendicular respectively to one edge of the throat of thehorn and of like phase, the throat and mouth of the horn being ofdifferent rectangular shapes and the dimensions of the horn beingrelated to A in such a manner that, in operation, one of the waves intravelling from the throat to the mouth of the horn is delayed byapproximately nx/4 relatively to the other wave where n is an oddinteger the ratio of the height to the width dimension beingcontinuously variable from the throat to the mouth of the horn.

2. An electromagnetic wave radiator according to claim 1, wherein thethroat of the horn is of square crosssection and the smaller dimensionof the mouth of the horn is equal to the length of one edge of thethroat.

3. An electromagnetic wave radiator according to claim 2, wherein themeans for feeding the two plane polarised waves into the throat of thehorn, comprise means to feed a plane polarised wave polarised at 45 toone edge of the throat into the throat, the last said wave beingresolved into the two waves polarised substantially parallel andperpendicular respectively to one edge of the throat.

4. An electromagnetic wave radiator according to claim 3, wherein themeans for feeding into the throat of the horn the plane polarised wavepolarised at 45 to one edge of the throat are adjustable to enable theplane of polarisation of the wave to be made parallel to one edge of thethroat of the horn.

5. An electromagnetic wave radiator according to claim 4, wherein themeans for feeding into the throat of the horn a plane polarised wavepolarised at 45 to one edge of the throat of the horn, comprises awaveguide of circular cross-section connected between the throat and afurther waveguide of rectangular cross-section, the two waveguides andthe horn having a common axis.

6. An electromagnetic wave radiator according to claim 5, wherein thewaveguide of rectangular cross-section is angularly adjustable about thesaid axis.

7. An electromagnetic wave radiator according to claim 5, wherein thewaveguide of circular cross-section is angularly adjustable about thesaid axis and contains a References Cited in the file of this patentUNITED STATES PATENTS Tyrrell Mar. 27, 1951 Fox June 10, 1952 Bowen July15, 1952 Purcell et al. Aug. 19, 1952 6 Alford Sept. 16, 1952 King June29, 1954 Hershfield July 24, 1956 Barnett Oct. 28, 1958 Crandcll et a1.Oct. 28, 1958 FOREIGN PATENTS Great Britain Nov. 29, 1946 Great BritainJan. 16, 1952

